The range and data rate of wireless internet services, as well as other forms of wireless data communications, depend on power, antenna gain, and signal bandwidth, among other factors. All three factors are limited both by economic and size considerations; furthermore, in the most commonly used frequency bands for unlicensed wireless internet services in the US, the 2400-2483.5 MHz ISM (industrial, scientific, and medical) band, as well as in the other unlicensed bands (e.g. 5725-5850 MHz), the transmitter power, transmitting antenna gain, and signal bandwidth are all directly or indirectly limited by federal regulations (Title 47, Part 15, Sec 15.247).
Current regulatory limits for point-to-multipoint communications (e.g. the base to client link when a base serves multiple clients) in the above mentioned bands require spread spectrum operation covering most of the frequency band, and an EIRP (effective isotropic radiated power) of no more than 36 dBm with a transmit power of no more than 30 dBm. Thus systems taking full advantage of the allowable parameter range need an antenna gain of at least 6 dBi. Systems with lower power transmitters need a higher antenna gain, for example, a 20 dBm transmitter is best operated a 16 dBi antenna. Current commonly used solutions for low gain (6-12 dBi) antennas in the ISM band are collinear verticals and corner reflector antennas. Common medium gain antennas (12-20 dBi) are arrays of dipoles and patches, with or without corner reflectors or backplates. For high gain antennas (>20 dBi) parabolic reflectors are almost exclusively used.
The option of reduced transmit power with increased gain is desirable from a point of view of interference reduction, and also reduces the transmitter/power amplifier cost. On the other hand, end users generally find smaller antenna size desirable, both for appearance, mounting, and safety concerns. Furthermore, lower gain antennas are simpler to align and less critical in their mounting accuracy.
The present invention addresses the need for antennas with reasonably high gain (12 to 24 dbi) that have reduced size, both in terms actual volume and in visual size as perceived from a distance, and greater ease in alignment and mounting, while still covering the entire required frequency range. Since electromagnetic principles show that smaller antennas generally have smaller gain and reduced bandwidth, innovative design techniques are needed to achieve a size reduction without impacting performance.
Furthermore, for a particular value of antenna gain, a fan beam with a narrow beamwidth in the horizontal plane and a relatively broad beamwidth in the vertical plane is desirable for three reasons. First, inteference sources/receptors have a tendency to appear distributed along the horizon as seen from the antenna. A narrow beamwidth in the horizontal plane will have significantly improved ability to discriminate between interference sources/receptors and the desired link, while the broad vertical beamwidth will sacrifice little in this respect. Second, having a broad beam in one plane means that accurate pointing is necessary only in the other plane. Thus, a greatly simplified mounting structure with only one degree of freedom is possible, improving both cost and rigidity. Third, since only one degree of freedom is available in the mounting initial alignment when the antenna is installed is simplified.
The present invention employs techniques including antenna folding, dielectric loading and end loading in a printed circuit format in order to reduce the size of the antenna, in particular the height when used as a vertical polarization radiator. The gain is achieved by employing both Yagi-Uda and broadside array techniques. The array configuration also yields a beam that is narrower in the horizontal plane than in the vertical plane. The combination of reduce size, ease of mounting, and interference reduction should be attractive and useful, particularly for client stations in a situation where multiple clients communicate with a base station.